The heart is a mechanical pump that is stimulated by electrical impulses. The mechanical action of the heart results in the flow of blood. During a normal heartbeat, the right atrium (RA) fills with blood from the returning veins. The RA then contracts and this blood is moved into the right ventricle (RV). When the RV contracts it pumps the blood to the lungs. Blood returning from the lungs moves into the left atrium (LA) and, after LA contraction, is pumped into the left ventricle (LV), which then pumps it throughout the body. Four heart valves keep the blood flowing in the proper directions.
The electrical signal that drives the mechanical contraction starts in the sino-atrial node, a collection of specialized heart cells in the right atrium that automatically depolarize (change their potential). The depolarization wavefront passes across all the cells of both atria and results in atrial contractions. When the advancing wavefront reaches the A-V node, it is delayed so that the contracting atria have time to fill the ventricles. The depolarizing wavefront then passes across the ventricles, causing them to contract and to pump blood to the lungs and body. This electrical activity occurs approximately 72 times a minute in a normal individual and is called normal sinus rhythm.
Abnormal electrical conditions can occur that can cause the heart to beat irregularly; these irregular beats are known as cardiac arrhythmias. Cardiac arrhythmias fall into two broad categories: slow heart beats or bradyarrhythmia and fast heart beats or tachyarrhythmia, clinically referred to as as bradycardia and tachycardia, respectively. Bradycardia often results from abnormal performance of the AV node. Stimuli generated by the heart's own natural pacemaker, the SA node, are improperly conducted to the rest of the heart's conduction system; as a result, other stimuli are generated although their intrinsic rate is below the SA node's intrinsic rate. Clinical symptoms associated with bradycardia include lack of energy, dizziness, etc., as the heart beats more slowly than is usual.
Bradycardia has been treated for years with implantable pacemakers. Their primary function is to monitor the heart's intrinsic rhythm and to generate a stimulus strong enough to initiate a cardiac contraction in the absence of the heart's own intrinsic beat. Typically, these pacemakers operate in a demand mode in which the stimulus is applied if the intrinsic rhythm is below a predetermined threshold.
Tachycardia is often associated with cardiac fibrillation, a condition in which the electrically coordinated aspects of the cardiac wave fronts are lost and, instead, have degenerated into chaotic, almost random electrical stimulations of the heart. Tachycardia often results from ischemic heart disease in which local myocardium performance is compromised As a result of tachycardia, coordinated contraction of heart tissue is lost which leads to a loss of blood flow to the rest of the body. Brain death can occur within several minutes of tachycardia, followed by complete death several minutes later if the tachycardia is left untreated.
Application of an electrical stimulus to a critical mass of cardiac tissue can be effective in extending the refractory aspects such that the heart can recover from its chaotic condition and resume normal coordinated propagation of electrical stimulation wave fronts that result in the resumption of normal blood flow. Thus, the application of an electrical stimulus can revert a patient's heart to a sinus cardiac rhythms and the chambers of the heart once again act to pump in a coordinated fashion. Such a stimulus is known as defibrillation.
Cardioversion/defibrillation is a technique employed to counter arrhythmic heart conditions including some tachycardias in the atria and/or ventricles. Typically, electrodes are employed to stimulate the heart with high energy electrical impulses or shocks, of a magnitude substantially greater than the intrinsic cardiac signals. The purpose of these high energy signals is to disrupt the generation of the chaotic cardiac signals and cause the heart to revert to a sinus rhythm.
There are two kinds of conventional cardioversion/defibrillation systems: internal cardioversion/defibrillation devices, or ICDs, and external automatic defibrillators, or AEDs. An ICD includes a housing containing a pulse generator, electrodes and leads connecting the electrodes to the housing. The electrodes are implanted transvenously in the cardiac chambers or are attached to the external walls of the heart. Various structures of these types are disclosed in U.S. Pat. Nos. 4,603,705, 4,693,253, 4,944,300, 5,105,810, 4,567,900 and 5,618,287, all incorporated herein by reference.
In addition, U.S. Pat. Nos. 5,342,407 and 5,603,732, incorporated herein by reference, disclose an ICD with a pulse generator implanted in the abdomen and two electrodes. In one embodiment (FIG. 22), the two electrodes 188, 190 are implanted subcutaneously and disposed in the thoracic region, outside of the ribs and on opposite sides of the heart. In another embodiment (FIG. 23), one electrode 206 is attached to the epicardial tissues and another electrode 200 is disposed inside the rib cage. In a third embodiment (FIG. 24), one electrode 208 is disposed away from the heart and the other electrode 210 is disposed inside the right ventricle. This system is very complicated and it is difficult to implant surgically since it requires three separate incisions.
Recently, some ICDs have been made with an electrode on the housing of the pulse generator, as illustrated in U.S. Pat. Nos. 5,133,353, 5,261,400, 5,620,477, and 5,658,325, all incorporated herein by reference.
ICDs have proven to be very effective for treating various cardiac arrhythmias and are now an established therapy for the management of life threatening cardiac rhythms, such as ventricular fibrillation. However, commercially available ICDs have several disadvantages. First, they must be implanted using somewhat complex and expensive surgical procedures that are performed by specially trained physicians. Second, they rely on transvenous leads for the placement of at least one electrode within the cardiac chambers. It has been found that over a period of time the electrodes get dislodged from the cardiac tissues, undesirable tissue formations may deposit on the electrodes, or the leads can break. These problems are especially acute when leads carry two or more electrodes. Third, removing these ICDs and replacing them, if necessary, also requires complicated surgical procedures that may be more life-threatening than the initial implantation.
As mentioned above, AEDs are also employed to provide antiarrhythmic therapy. A typical AED is similar to an ICD in that it also includes a pulse generator and a pair of electrodes connected to the pulse generator by leads. Because all of these elements are external, they can be made larger than the corresponding elements of an ICD. Moreover, because the electrodes are applied externally, an AED typically requires more power than an ICD.
Three types of AEDs are presently available. One type is normally used in a hospital or similar facility and is designed to be used by a trained physician when a patient is suffering from an acute tachyarrhythmia.
The second type is placed in public places such as theaters, airports, and so on, and is designed to be used in an emergency by people with less training, such as medical technicians, or even lay persons. Both the first and second types of AED require some kind of intervention by a person before operation.
A third type of AED has been proposed which is adapted to be worn by a patient, and which could function without an operator.
All three types of AEDs require the application of external electrodes which are uncomfortable. Even the third type of AED presents at best only a short term solution to cardiac arrhythmia.